28th International Cosmic Ray Conference

3957

Studies on Cosmic Ray Sidereal Anisotropy with
the Multidirectional Muon Telescope at Ooty
H. Kojima,1 K. Fujimoto,1 S.K. Gupta,2 Y. Hayashi,3 Y. Ishida,3 N. Ito,3Atul Jain,2 S. Kawakami,3 T. Nonaka,3 A. Oshima,3 K. Sivaprasad,2 S. Tamaki,3H. Tanaka,3 S.C. Tonwar2 and T. Yoshikoshi3 (The GRAPES Collaboration)
1. Nagoya Women’s University, Nagoya 467-8610, Japan
2. Tata Institute of Fundamental Research, Mumbai 400005, India
3. Graduate School of Science, Osaka City University, Osaka 558-8585, JapanAbstract
We have developed a multidirectional telescope capable of recording individual muons with angular accuracy of about 5 degrees. This muon telescopeconsists of ∼ 3000 proportional counters with total area of ∼ 420 m2 with threshold energy > 1 GeV. The telescope is a component of the GRAPES-3 experiment
at Ooty in southern India (N 11.4, E 76.7 and 2200m altitude). The very large
muon counting rate ∼ 1.8×108 per hour, achieved due to the very large area of thetelescope, gives us great advantage for cosmic ray modulation studies. The analysis of data with such high statistics enables us to have a sensitive measurement ofsidereal variation within a single year of observation. Further, since our telescopeis located near the Equator, we are able to observe both the Northern and theSouthern hemispheres simultaneously. We present here the results on the siderealvariation obtained with this multidirectional muon telescope for the observational
period, 2000–2001. We report here on Tail-in and Loss-corn anisotropies throughdetailed analysis. We also discuss other possible explanations for the present
observations.
1.

Introduction

Many causes are contributing for the variation of gallactic cosmic rays
intensity variation energy above around 10 GeV.[1][3][4][5][6][7] Though there
might be some eﬀect arising from the solar magnetic ﬁeld ranging greater than
100 AU[1], major contribution would come from the outside of it. That is gallactic
space origin. So far we are not able to point out the exact cause of anisotropy insidereal daily variation of Cosmic Rays.
Nagashima and Fujimoto et. al. claimed that they have observed two typeof anisotropy called Loss Corn and Tail in with the results from several observation, Nagoya, Sakashita, Norikura, Hobart.[4][6] They proposed that Loss corn
defect should occur due to the gallactic magnetic ﬁeld around the Solar systemscpp. 3957–3960 2003
by Universal Academy Press, Inc.

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Fig. 1. a Cross sectional view of multidi- Fig. 1. b Schematic figure for combined
rectional muon telescope
directionand excess ﬂux of cosmic rays can be expected from Tail in.[4][6][7] According
to them there must be bow shock which can be formed with the interaction between solar magnetic ﬁeld and inter stellar plasma. If the bow shock is formed,
it becomes easy to enter from opposite direction, toward the Tail of the magneticﬁeld. The energy of their data is ranging from 10 GeV to 104 GeV.[6][9]According to them Loss Corn defect exist in direction of right ascension
h12 , declination 20 degrees and Tail in 6h , declination -16 degrees. Since our muon
telescopes are located near Equator (N 11.4, E 76.7), we have a great advantageof looking them simultaneously.[10]2.

Data analysis and results

We have collected multidirectional muon intensity data from 1999 to 2001.
3 units of telescope were operational in 2000 and 12 units were operationalin 2001. Our multidirectional muon telescopes can observe 15×15 = 225 direction
simultaneously. In order to achieve better statistics the directional muon data iscombined for 9 direction. Each direction is named as follows, NW, N, NE, W, C,E, SW, S, SE and they are shown in Fig. 1b.
The procedure of the analysis is described in details elsewhere in this conference. In order to eliminate the common error ﬁrst we made the diﬀerential
data as follows. The data coming from westward is subtracted from the datafrom eastward and we have reconstructed the sidereal hourly variation for one
day with integrating this diﬀerential data.[2][4]We have categorized the muon data according to the polarity of Interplanetary Magnetic Field, Toward and Away from the Sun. The polarity of each day
was determined from the satellite data of ACE level2. Calculating the 72hourmoving average we adopted the value at 20h UT for corresponding day’s polarity.3959

Fig. 2.

Fig. 3.

Sidereal time variation of Cosmic rays (Away)

Sidereal time variation of Cosmic rays (IMF: Toward)

One can clearly see typical sine wave like shape of hourly sidereal time
variation of cosmic rays with the IMF polarity Away from the Sun in Fig. 3.
Since this anisotropy comes from so called Swinson ﬂow, it is expected to have apeak in 6h in case of Away and same kind of variation is expected for Toward also
having a peak in 18h from same Swinson ﬂow.[1] But the variation in Toward’
data does not look smooth and nice. The reason why this smearing of peak in
Toward’s data happened is not clear at present. It may have some connection with
the north-south asymmetry in solar time variation of muons which we report inthis conference. Or it could be due to the strong IMF, since the activity of Sun isnearly at maximum around 2000 and 2001. To deduct this pseudo-sidereal eﬀect
we have made the average of Away and Toward data. Please see Fig. 4. We don’t
see clear Tail in anisotropy at 6h and Loss corn at 12h in Fig.4. This might bedue to the strong activity of Sun and its IMF. If the intensity of IMF is strong,incoming cosmic rays might be scattered and then the peak would have become
ﬂat. We would like to conﬁrm present result with continuing this observation up
to quiet season of the Sun’s activity.

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Fig. 4.

3.

Sidereal time variation of Cosmic rays (Toward+Away)/2

Acknowledgement

We are thankful to Japan Society for the Promotion of Scieince for partial
ﬁnancial support for this experiment. We are also happy to acknowledge valuable
contributions of G. Paul Francis, K. Manjunath, B.S. Rao, K.C. Ravindran, V.
Viswanathan and T. Matsuyama during the installation, operation and maintenance of the instrumentation. The help and cooperation of the Radio Astronomy
Center for providing site facilities for the GRAPES III array are gratefully acknowledged.
4.

References(1) SwinsonJD.B, J. Geophys. Res., 745591,1969(2) Nagashima, K.,et al., Nuovo Cimento, C(6), 550. 1983a(3) Alexeenko, V.V., and Navara G., Lett Nuovo Cimento, 42(7), 321., 1985
(4) Nagashima, K. and Fujimoto, K.,et al., Nuovo Cimento, 12C(6), 695., 1989
(5) Bergamasco, L., Proc 21st Int Cosmic Ray Conf., Adelaide 6, 372–375.,1990(6) Nagashima, K., Fujimoto, K., and Jacklyn, R.M., J. Geophys. Res., 103, A8,17429., 1998
(7) Munakata K.,et al., Proc 26th Int Cosmic Ray Conf., SaltLake City, 7, 293–296., 1999
(8) Munakata K.,et al., Proc 27th Int Cosmic Ray Conf., Hamburg, 10, 3919–3922., 2001
(9) Fujimoto K.,et al., Proc 27th Int Cosmic Ray Conf., Hamburg, 10, 3927–3929.,
2001(10) Kojima H.,et al., Proc 27th Int Cosmic Ray Conf., Hamburg, 10, 3943–3945.,
2001